Lei Zhang (1), Shuai Chen (1), Cun Zhang (2), Xinqiu Fang (1), Shuang Li (3)

Fuel, 262, 15 February 2020. DOI: 10.1016/j.fuel.2019.116635

Keywords

Coalbed methane; Bituminous coal; Liquid nitrogen; Microstructure; Permeability; µCT

Abstract

Studies of the microstructure and permeability evolution behavior of coal under conditions of liquid nitrogen (LN2) fracturing are helpful to clarify the mechanism of fracturing. The evolution of the 3D microstructure of thebituminous coal before and after LN2 fracturing was studied, using a high-resolution 3D X-ray microanalyzer and 'Dragonfly' software. The pore-fracture structure was preliminarily segmented using a digital terrain model (DTM) threshold segmentation and then was optimized using a 'Deep learning' model. The box-counting method and a pore network model (PNM) were used to analyze the fractal features, spatial size distribution, and connectivity of pore-fractures. The results of the fractal analysis showed that the 2D fractal dimension of porefracture in the horizontal and vertical directions increased significantly after LN2 fracturing. The PNM analyses showed that the number and size of equivalent pores and throats after fracturing were significantly higher than prior to fracture. The relatively large fractures play a decisive role in the coal permeability, although they account for a small proportion of the total number of fractures. The results of the permeability tests showed that LN2 fracturing can effectively destroy the original structure of the coal and improve its permeability. At a constant confining pressure, the permeability and permeability increment increased exponentially with increasing gas pressure. The permeability increment curve can be divided into two stages: micro-fracture seepage and large-fracture seepage. During the first phase, the incremental change in permeability is small. During the second stage, the permeability increment increases sharply due to the re-opening of micro-fractures and the formation of large fractures.

How Our Software Was Used

Dragonfly was used to quantify digitally and characterize a coal pore-fracture structure, to segment 3D pore-fracture networks of coal samples and to preprocess, segment, and perform the quantitative analysis of CT images. Our software's window leveling tool was used to enhance the contrast of different CT scans to make the details clearer and to divide the pore-fracture of coal samples before and after LN2 fracturing. Its 'Deep Learning' tool allowed the optimization of DTM segmented images. Finally, Dragonfly's PNM module was used to extract the PNM of a pore-fracture structure.

Author Affiliation

(1) Key Laboratory of Deep Coal Resource Mining, Ministry of Education of China, School of Mines, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China
(2) School of Resource and Safety Engineering, State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (Beijing), Beijing 100083, China
(3) School of Management, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China